1. Field of the Invention
The present invention relates generally to an active-matrix addressed liquid crystal display (LCD), and more specifically to such an LCD having a reflective electrode via which ambient light as the reading source is reflected thereby and again emitted out of the device. Still more specifically, the present relates to a method of fabrication such an LCD.
2. Description of Related Art
LCDs have found extensive uses in a variety of electronic devices such as television receivers, personal computers, personal digital assistances (PDAs), mobile telephone terminals, picture monitors, and so on. Among others, active-matrix addressed LCDs have widely utilized, which are provided with a plurality of active elements (switching elements) respectively assigned to pixel electrodes for controlling application of voltages thereto. The active element is typically a thin film transistor (TFT) or a metal-insulator-metal (MIM) diode. The active-matrix addressed LCD has distinct features of high resolution, a wide viewing angle, a high contrast, multi-gradation, etc.
The active-matrix addressed LCDs are generally classified into two types: one is transmissive (backlit) and the other is reflective. Although the transmissive LCD has many advantages, it has encountered the problems resulting from the presence of a built-in light source. Namely, the transmissive LCD undesirably becomes bulky and consumes considerable power. Therefore, in the case where low power consumption is needed such as in mobile telephone terminals, it becomes a current tendency to use a reflective LCD. The reflective LCD is provided with a reflective electrode for reflecting ambient light in order to enhance reflectivity, it is a common practice to provide fine roughness on the reflective electrode.
Before turning to the present invention, it is deemed preferable to briefly describe, with reference to FIGS. 1 to 2J, the conventional technology relevant to the present invention.
FIG. 1 is a schematic cross section showing approximately one pixel area of an active-matrix addressed reflective LCD using a single polarizing plate. The LCD shown in FIG. 1 generally comprises an upper substrate (or opposite substrate) 1, a lower substrate (TFT substrate) 7, and a liquid crystal layer 14 provided between the two substrates 1 and 7. As shown, the upper substrate 1 is comprised of a polarizing plate 2, a phase shifting plate 3, a glass substrate 4, a color filter 5, and a transparent electrode (common electrode) 6. On the other hand, the lower substrate 7 is comprised of a glass substrate 8, a thin film transistor 9 formed on the glass substrate 8, a first insulation layer 10 with unevenness or irregularity on the surface thereof, a second insulation layer 11 which is formed on the first insulation layer 10 and is made of polyimide, and a reflective electrode (reflective layer) 13. The thin film transistor 9 operates as a switching element. The reflective electrode 13, which is typically made of aluminum, is coupled to a source electrode 12 and functions as a pixel electrode in addition to reflecting ambient light. A liquid crystal layer 14 is sandwiched between the two substrates 1 and 7.
As shown in FIG. 1, incoming ambient light, schematically denoted by reference number 15, passes through the upper substrate 1, being reflected by the coarse-surfaced reflection layer 13, and returning to external environments as schematically shown by reference numeral 16.
In order to improve reflectivity at the reflective electrode 13, it is vital to make the upper surface of the electrode 13 uneven or irregular so as to effectively reflect incident light with various incident angles. The conventional irregularity appearing on the surface of the reflective electrode 13 is formed on the basis of a plurality of small hemispheres independently, randomly provided on the glass substrate 8.
Referring to FIGS. 2A-2J, a method of fabricating the above-mentioned conventional reflective electrode 13 will be described.
A gate electrode 21 is formed on the glass substrate 20 (FIG. 2A). Thereafter, a gate insulation film 22, a semiconductor layer 23, and a doping layer 24 are successively formed on the glass substrate 20 (FIG. 2B), after which an island 25 is formed by patterning the doping layer 24 and the semiconductor layer 23 (FIG. 2C). Subsequently, after a metal layer is deposited on the surface of the layer formed in FIG. 2C, a source electrode 26 and a drain electrode 27 are formed by patterning (FIG. 2D). Thus, the thin film transistor 9 is obtained.
Following this, as shown in FIG. 2E, a photo-sensitive organic insulation layer 28, which is of acrylic type photoresist (for example), is deposited on the surface of the resultant substrate obtained at the preceding step of FIG. 2D. Subsequently, a plurality of projections 29 are patterned by photolithography in the area above which the reflective electrode is to be formed (FIG. 2F), after which the projections 29 are heat-treated so that the angular parts thereof are smoothed, as shown in FIG. 2G. Thereafter, the smoothed projections 29 are covered by an organic insulation film 31, and accordingly, the surface of this film 31 exhibits smoothed irregular surface (FIG. 2H). Then, a contact hole 33 is formed (FIG. 21), after which the reflection electrode 34 is deposited, as shown in FIG. 2J, on the surface of the resultant substrate obtained the preceding step of FIG. 21. Thus, the source electrode is electrically connected to the reflective electrode 34. The above-mentioned technique is disclosed in Japanese Post-Examination Patent Application No. 61-6390.
However, the active-matrix addressed reflective LCD according to the above-mentioned conventional technique has suffered from the following difficulties.
First, the photo-sensitive insulation layer 28 is patterned by photolithography, in the case of which a low-sensitive photoresist such acrylic type is typically selected in order to obtain fine adjustment of irregularity. Therefore, the conventional technique suffers from the problems that the intensity of exposing light should be increased and the exposure time becomes long, which renders the fabrication processes complicated and undesirably increase the fabrication time of the device.
Second, the storage capacitance of each pixel is small, and thus, it is liable to induce flicker. In order to increase the storage capacitance, it is conceivable to increase overlapped area of the gate line and the reflective electrode (viz, pixel electrode) as implemented in a transmissive LCD. However, since each of the organic insulation layer 28 and the polyimide layer 11 has inherently a considerably large thickness in the case of an active-matrix addressed reflective LCD, and as such, it is difficult to realize sufficient storage capacitance.
Third, the projections 29 shown in FIG. 2G have typically diameters approximately ranging from 1 to 20 μm and heights approximately from 0.5 to 5 μm and are independently formed on the gate insulation layer 22. Accordingly, the projections 29 are liable to peel off from the gate insulation layer 22 during the substrate processing of washing, heat treatment, film deposition, etc. As a result, it is difficult to achieve the expected irregularity on the surface of the reflection electrode 34.